Control system for once-through flow vapor generator



Aug. 24, 1965 R. D. HoTTENs'rlNE CONTROL SYSTEM FOR ONCE-THROUGH FLOW VAPOR GENERATOR Filed Dec. 28, 1962 ATTORNEY n is `forced at supercritical pressure.

United States Patent O f This invention relates generally to forced once-through j flow generators'pand has particular relation to an irnt proved control system for such a generator whereby the linal vapor temperature of the generator is maintained constant notwithstanding that the load on the generator varies. `In forced once-through iiow vapor generators the working medium is forced, by suitable pump means, through `whatjis termed the through ow'circuit of the vapor generator. Such vapor generators include both supercritical and subcritical operation with the vapor generator shown and describedin U.S. application No. 127,395 of W. W. Schroedter tiled July 27,1961 and assigned to the same assignee as` `the assignee of the instant application, being an example of a modern supercritical vapor generator throughthe through How-circuit of which the feedwater The control system ofthe present invention employs a desuperheater is the superheating portion of the through flowsystem which has for its purpose the rather rapid correction of transient variation in the steam temperature. The steam temperature` as well as the pressure of t-he vapor issuing from the once-through ilow vapor generator is controlled primarily by suitably regulating the flow of `feedwater to and through the supercritical circuit and the tiring of fuel in the furnace which forms a part of the vapo-r generator. By suitably regulating the ratio of feedwater and fuel or tiring rate, `the pressure and temperature of the vapor egressing from .the through iiow vapor generator may be regulated throughout ,a predetermined load range. However, since the vapor generators to which the invention pertains are very massive structures, the regulation that is obtained in this manner :has` a considerable `time delay which, if nol other control action were provided, would result in con- `siderable overshooting and undershooting of the desired temperature. The desuperheater as employed with the in- `vention is operated during normal operation of the kvapor generator, intermediate -of its upper and lower eXtreme limits so that it may quickly effect a decrease or an increase in the temperature of the vapor in the through Vflow circuit as required to` maintain the final vapor ternperature at itsdesired value. The desuperheater is accordingly effectively controlled in response to the final `vapor `temperature and when this temperature varies from its desired value, `the desuperheater will be operated in response to` this variation in a manner to cause this temperature to return to its` desired value.

l lnaorder that the` desuperheater will bemaintained,

` during normal operation, `at a` level of operation` inter- `mediate its upper and `lowerextreme limits, the temperap ture' diiferencein the through flow iiuid immediately upstrearn and immediately downstream of the desuperheater final temperature to return to its desired value with the time required before this adjustment of the through flow and tiring is felt at the output of the vapor generator being greater than the corresponding time for the desuperheater. This change in the desuperheater operation to provide a rapid correction of the vapor temperature resuits in the temperature difference across the desuperheater also changing. ,This in turn causes a readjustment of the ratio of the through flow to the firing rate which in turn affects the nal temperature of the vapor generator and accordingly the action of the desuperheater withtthe ultimate effect being that the final `temperature is maintained generally at its desired value and the temperature `difference across the desuperheater is returned is capable of correcting other sudden transient variations ofthe final temperature of the `vapor regardless of whether an increase or a decrease in the desuperheating Vcapacity is required to effect the correction.

Accordingly, it is an object of the present `invention to provided a forced through flow vapor generator with an improved control organization.

A further objectof the inventionis to `provide a `forced through flow vapor generator provided with a desuperheater and wherein the temperature difference across the desuperheater in utilized as a control parameter.

Other and further objects of the invention will become apparent to those skilled in` the art as the description proceeds.

With the aforementioned objects in` view, the invention comprises an arrangement, construction and combination of the elements ofthe inventive organization in such a manner as to attain the results desired as hereinaftermore particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein the single figure is a diagrammatic representation of a 'forced through flow vapor generator employing the improved control system of the invention. i t

Referring now to the drawing, wherein like reference characteristics are used throughout to designate like `elements, the diagrammatically represented force through iiow vapor generator, which maybe'either supercritical or subcritical, includes the feedpump lii which forces the feedwater through `the flow measuring device 12 and the How control valve means ift into and through the economizer 16. From the economizer the vaporizable Working fluid ows through the heating surface 1S and from this heating surface the fluid flows through the connecting `conduit 20 to and through thetinal or `finishing heating surface 22. Upon egressing fromA this final heating surface, the fluid is in vapor form with the nal temperature and pressure of this vapor `being at a desired` Value for delivery toa prime mover, such as a turbine, with the iiuid passing through the valve 24 which 4may be the turbine `valve andthen to the primemover whichIis not illustrated `is `utilized to adjust the ratio of feedwater flow through the through ow circuit to the firing of the vapor generator with this adjustment or regulationy being such as to Amaintain `this.temperature diiferenceat a predetermined valve. Accordingly, the control system operates so that `as a result of a change in the final `temperature of the vapor of the vapor generator from `its Vpredetermined valuepthe'desuperheater is operated` solas toeffect a rather 1 rapid return of this temperature to its desired value. At the same time the iow of fluid through the vapor generatoraand the tiring rate is `adjusted so as to cause the and which in a conventional `system drives an electric generator.v As is conventional in power plane systems, the vaporafter leaving the prime `mover' 1s condensed,

heated by`v` suitable preheaterade-aerated and returned to `the through flow circuit, being forced againtthroughthis circuit by means of the feedpump` means lili` s f i In' the illustrative organization heat is supplied to th vapor generator by means of the tiring means 26 `whichis supplied with air through the duct 28 and regulating means 3i? and is supplied with fuel through the conduit 32 and the regulating means 3d.

After traversing the economizer 16 and the heating surface 18, the working uid of` the `vapor generator `is in vapor form and partially superheated to its desired tem- 3 perature. In passing through the conduit Zh to the iinal superheating surface 22, the amount of this superheating is reduced by means of the attemperator or desuperheater 36 that is connected into this conduit Ztl. In the illustrative arrangement this desuperheater is of the spray Vor direct'contact type wherein relatively cool Water, which 'may be feedwater or other water substantially pure to be introduced into the through low system of the vapor generator, is sprayed into the superheater vapor traversing the conduit 20. The purpose of the desuperheater is to rprovide what may be termed a transient control for the final vapor temperature of the vapor generator or in other words to eifect a rather rapid transient, temperature correction. In order that the desuperheater may eilect either 'an increase or decrease in the inal vapor temperature,

it is necessary that, during normal operation of the vapor generator, the desuperheater operate intermediate its upper and lower extreme limits. The lower limit may be `that obtained when there is no flow of the cool desuperheating uid to the desuperheater 36 while the upper limit is that obtained when'the maximum flow of the cool desuperheated fluid is provided through the desuperheater.

The desuperheating uid is conveyed to the desuperheater through the conduit 33 with the regulation of this iluid -ow being obtained by means of the regulator du.

It will be understood that the desuperheater may take a dierent form other than the direct contact spray de- -superheater hereinbefore described with the invention including other types of desuperheaters, such as the indirect heat exchanger type of desuperheater.

While the desuperheater, as mentioned hereinbefore, is for the purpose of providing a relatively rapid correction for transient variations in temperature of the vapor egressing from the vapor generator, the primary ycontrol for this temperature is obtained through adjustment of the feedwater flow through the vapor generator as well as the firing of the vapor generator with the relatively quick acting-desuperheater control action being desired because of the inherent rather substantial 'time lag in the massive vapor generator between adjustment of the feedwater iiow and the tiring and the eect of this adjustment being evidenced in the conditions of vthe vapor egressing from the vapor generator.

The pressure of the vapor leaving the vapor generator is sensed by the pressure sensing device 42 with the de- 'vice 42 providing a signal which is compared with a desired setpoint signalat the summation point 44. This 'comparison will provide an error signal ifthe pressure leratorthat is driven by the turbine supplied with vapor from the vapor generator, and a signal representing the actual megawatt output. Any difference between these two values provides an error signal which is fed to the :controller 52. The signal resulting from the comparison of the output signal of controller 48 and the output siglnal of controller 52 at summation point 50 is employed to Vregulate the valve 24 controlling the supply ofvapor to the prime mover. The output of controller 46 and the output of controller SZare combined at summation point 56 with this signal in turn being employed for regulating the feedwater low and the tiring.

The temperature of the vapor leaving the vapor generator is sensed by the temperature sensitive device S3 which provides a signal that is compared with a desired set point at the summation point 60. The resulting signal is conveyed as the input to the controller 62. and the ycontroller 64 with this signal being a temperature error signal or in other words a signal representing the variation of the temperature (T3) from its desired value or 4 set point. The output of the controller 64 is combined with the output signal from summation point 56 at the summation points 66 and 68. The output signal from summation point 63 is operative to regulate the air ilow regulating means 3@ and the fuel flow regulating means 3ft while the output of summation point 66, which provides a signal that represents the desired feedwater ilow, is compared at summation point 72 with a signal produced by the flow responsive device 70 representing the actual flow. The output signal from summation point '72 represents ow error which is the input of the controller 74 with this controller in turn regulating the flow control valve means M to adjust the `flow of fdeedwater to and through the through flow circuit of the vapor generator.

The output of the controller 62 provides a signal which acts as a set point for the temperature T2 immediately downstream of the desuperheater with T2 being the temperature of the through ilow at this location. The tem'- perature sensor device 76 responds to this temperature and provides a signal representative thereof and which is compared at the summation point '78 with the output signal of the controller 62 or in other words the set point and with this comparison providing an error signal Vwhich is conveyed as the input to the controller Si). The output signal of controller St) is operative to adjust the regulator 49 for admission of the desuperheater fluid to the desuperheater 36. Accordingly as a result of a change in the temperature T 3, an error signal is supplied to the controller 80 which in turn causes a readjustrnent of the regulator 40 with this operation being such as to cause the temperature T3 to return to its desired value. However, also as a result of this temperature T3 varying from its desired value, an error signal is supplied to the controller 64 the output of which is effective to cause a readjustment of the ratio of the tiring of the vapor generator and the feedwater flow thereto so as to return the temperature T3 to its desired value.

In order for the desuperheater 36 to be able to rapidly correct transient temperature variations including both an increase and a decrease in the nal temperature of the vapor of the vapor generator, it is necessary that the desuperheater be maintained, during normal operation, so that it is Within and well displaced from its upper and lower extremes 'of capacity. It no meanswere provided to insure that the desuperheater was maintained intermediate these ranges and preferably well displaced from leither extreme, the condition would soon exist where the desuperheater was at or 'close to one extreme or the other and accordingly could effect a transient correction only in one direction. To insure that such a condition does not exist, the temperature of the through flow immediately upstream and downstream or in other Words across ythe desuperheater 36 is sensed with the temperature sensticular desired temperature difference between T1 and j T2. The output of the summation point 86 represents a temperature difference error signal'with Athis signal being combined with the temperature error signal from summation point 6i) at junction 8S as the input of controller 64. Thus the temperature difference across the desuperheater is utilized to adjust the ratio of the feedywater flow and the tiring rate so as to maintain this temperature at its desired value. Accordingly when there is a rapid change in the temperature T3, such as may occur when slag suddently falls from the heating surface 22, the

`desuperheater 36 is effective to provide a rapid correction of this temperture while as a result of the variation in T3 the ratio between the feedwater and the tiring rate is adjusted to provide a primary or permanent correction.

`It is `noted `that the desuperheater will respond to a` change in temperature of the through flow lat the loca` tion of T2 and provide a corrective action even before this temperature change reaches the outlet of the vapor generator or in other Words the location of T3. Thus, should there be a sudden temperature change, as might be caused from slag suddenly being removed from the heating surface 18, this change in temperature will be detected by the temperature responsive device 76 which provides an indication of the temperature T2 and the signal thus produced will be compared with the set point signal produced by the controller 62 thereby producing an error signal which is received by the controller 8i) and which is effective to adjust the regulator di) so as to correct the upset in temperature. This change in the positioning of the regulator 4Q will, of course, change the temperature differential across the desuperheater, i.e., the difference in temperature between T1 and T2 which will in turn readjust the ratio of the through flow to the tiring rate so as to bring this temperature difference back to its desired value.

When the desuperheater is adjusted to effect the desired transient temperature correction, the temperature diiference across the desuperheater changes with this in turn causing a readjustment of the ratio of the feedwater ow and the iiring rate so as to tend to bring this temperature difference back to its desired value.` The over-al1 result is 4that the temperature T3 is rapidly corrected and the de- `superheater is eventually readjusted so that the temperature differential thereacross is returned to its desired value after it has performed its function of providing a transient correction.

In order to have the temperature differential across the desuperheater 35 returned to its predetermined value after an upset due to a transient temperature change in the system of the once-through how vapor generator, it is necessary that the ratio of the feedwater ow and the ring rate be adjusted for this purpose. This may be accomplished either by varying the feedwater per se or varying the tiring rate per se. However, in the illustrative system depicted in the drawing, the feedwater and tiring rate are varied simultaneously but in opposite directions. This control operation is provided by the controller 64 with the output of this controller effecting such a manipulation of the feedwater flow and firing rate.

The control system of the invention may be either electrical in nature or may behydraulic or may empioy air pressure. An electric system is preferred because of its convenience and the controllers indicated may be either controllers of the type manufactured by Leeds and Northrup Company or the Hagan Company or others which manufacture three-action type, i.e., providing proportional action, integrating action and differential action. As indicated, the controllers 48, 64, 74, and 80 need not provide or use the diiferential action since diiferential action here will be of no particular advantage. lt may be usedif desired, however. The summation points indicated `need no controlling hardware if an electrical system is used since they are merely wiring points where D C. voltage signals are combined or compared. The temperature, pressure and flow transmitters or transducers can be of the standard Leeds and Northrup Company or Hagan Company type with these transmitters transmitting a signal that is compatible with the controller inputs.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. A once through ow vapor generator comprising firing means and a through iiow circuit having a superheating portion, desuperheating means in said superheating portion operative to lower the temperature of the through flow in passing through this last-named means, adjustable means regulating the effectiveness of said desuperheating means, means responsive to the temperature of the through ow at a location downstream of the desuperheating means in the through flow circuit effective to regulate said adjustable means to correct transient temperature changesin said fluid and means responsive to the temperature difference of the through ilow across said desuperheating means and little or no superheating surface effective to regulate the ring rate and flow through said through ow circuit so as to maintain this difference at a generally predetermined value.

2. The organization of claim 1 wherein said desuperheating means is a direct contact spray desuperheater and said adjustable means is an adjustable control controlling the introduction of the desuperheating spray.

3. In a fuel iired vapor generator having a so-called through iiow circuit through which the vaporizable medium is forced by a pump being superheated to a desired temperature during traversal of the circuit and also having a desuperheater operative to control transient variations of the final temperature of the through flow fluid during operation the method comprising operating the desuperheater intermediate `and displaced well from its upper and lower limits of effectiveness during normal operation of the unit, regulating the temperature and pressure of the iiuid egressing `from the through flow circuit and de.d livered to the point of use through manipulation of the firing of the unit and the feed of fluid to the through iow circuit in direct response to deviations from a desired value, correcting transient variations in the temperature of the through iiow through regulation of the desuperheater intermediate said upper and lower limits, and` detecting the temperature difference across the desuperheater and `little `or no superheater surface, and adjusting the ratio of the through flow to the tiring rate to return this difference to a predetermined value. y

4. ln a once through flow vapor generator the combination of a through flow circuit, means forcing the vaporizable fluid through said circuit, means imparting heat to said fluid, means intermediate the extremities of said circuit operative to pass a lower temperature fluid in heat exchange relation with the through How to decrease the temperature of the latter, means responsive -to the inal temperature of the through flow directly operative to control the flow through the through dow circuit and the heat imparted thereto to maintain this temperature at a generally predetermined value, temperature responsive means being eiective to regulate the eifectiveness of the` means directing the lower temperature fluid in heat exchange relation with the through flow, and means responsive to the difference in temperature of the through flow entering the location where the lower temperature fluid is passed in heat exchange relation therewith and the temperature of the through-flow leaving the location where the lower temperature iiuid is passed in heat exchange relation therewith effective to regulate the flow through the through flow circuit and the heat imparting means to maintain this difference at a generally predetermined value.

5. A vapor generator comprising in combination tubu-` rectly regulating said desuperheater to thereby provide a relatively fast correction of steam temperature variations, means sensing the temperature diiierence across said desuperheater and little or no superheating surface and operative to adjust the ratio of the iiow of vaporizable uid and firing rate to maintain said difference at a generally predetermined value.

6. in a vapor generator supplying superheated vapor to a variable load and wherein iiuid is forced through heat exchange surface including superheater surface with there being fuel tiring means for generating and imparting heat to the fiuid traversing such surface, means providing for regulation of the vapor temperature egressing from the vapor generator for supply to the load including desuperheating means associated with the superheater surface of the unit and normally operating intermediate its upper and lower limits, means responsive to the temperature of the vapor egressing from the vapor generator operative to adjustably regulate said desuperheater to maintain this temperature at its desired value and also operative to regulate the iiow through the vapor generator and the tiring thereof to maintain this temperature at its desired value, and means sensing the temperature difference across the desuperheater and little or no superheater surface and in response thereto regulating the ratio of the flow through the vapor generator to the tiring rate to maintain this temperature difference at a predetermined value.

'7. In a forced through flow vapor generator tired with `a suitable fuel and producing superheated vapor, said Astant, measuringl the temperature of the through ow imvmediately upstream and immediately downstream of the ,desuperheater and in response to the difference in these measurements regulating the ratio of the flow through the vapor generator and the tiring rate so as to maintain the difference between thesetwo temperatures at a generally predetermined value.

S. The method comprising pumping a vaporizable uid through a circuit, burning a fuel and imparting heat to said iiuid during its traversal of said circuit thereby vaporizing said liuid and raising it to a predetermined inal degree of superheat, passing a lower temperature fluid in heat exchange relation with the vaporized fluid prior to the latter reaching its nal degree of superheat, providing a first control for maintaining said iinal temperature at its desired value including sensing said final temperature and in response thereto regulating the heat input to the vaporizable iiuid and the iiow of this iuid through the circuit and providing a second control of said temperature having a response time substantially less than said first and including adjusting the passage of the low temperature iiuid in heat exchange relation with said vaporizable fluid in response to the final temperature of the latter and readjusting the passage of this low temperature liuid in heat exchange relation with the vaporizable fluid after a change thereof by sensing the temperature difference in the vaporizable fluid immediately prior to and after the location where the low temperature uid is passed in heat exchange relation with the vaporized iiuid and in response to such temperature difference adjusting the ratio of the flow through the circuit and the rate of burning fuel to maintain such difference at a predetermined value.

9, The method of claim 8 wherein said lower tempera- 4ture iiuid is introduced into direct contact with said vaporized uid.

References Cited hy the Examiner UNITED STATES PATENTS 2,966,896 l/'61 Volger 122-479 3,086,503 4/ 63 Profes 122-448 FOREIGN PATENTS 217,051 9/61 Austria. 358,096 12/ 61 Switzerland.

PERCY L. PATRICK, Primary Examiner.

KENNETH W. SPRAGUE, ROBERT A. OLEARY,

Examiners. 

1. A ONCE THROUGH FLOW VAPOR GENERATOR COMPRISING FIRING MEANS AND A THROUGH FLOW CIRCUIT HAVING A SUPERHEATING PORTION, DESUPERHEATING MEANS IN SAID SUPERHEATING PORTION OPERATIVE TO LOWER THE TEMPERATURE OF THE THROUGH FLOW IN PASSING THROUGH THIS LEAST-NAMED MEANS, ADJUSTABLE MEANS REGULATING THE EFFECTIVENESS OF SAID DESUPERHEATING MEANS, MEANS RESPONSIVE TO THE TEMPERATURE OF THE THROUGH FLOW AT A LOCATION DOWNSTREAM OF THE DESUPERHEATING MEANS IN THE THROUGH FLOW CIRCUIT EFFECTIVE TO REGULATE SAID ADJUSTABLE MEANS TO CORRECT TRANSIENT TEMPERATURE CHANGES IN SAID FLUID AND MEANS RESPONSIVE TO THE TEMPERATURE DIF- 